The invention relates to helical gearing, more particularly worm gearing. The invention further relates to an electric power-assisted steering system comprising helical gearing.
Document DE 101 61 715 A1 makes known helical gearing of the type in question, which is embodied as worm gearing for use in an electric power-assisted steering system.
The worm gearing described in DE 101 61 715 A1 is disposed in a housing and comprises a worm and a worm gear. The worm is connected by way of a coupling to a drive shaft of an electric drive motor. In the design shown there, the worm is pivotable perpendicularly to the rotational axis and is supported in a rocker pivot or self-aligning bearing, thereby making it possible to hold the gear play constant during the entire service life of the gearing. To this end, the worm is acted upon radially by a preloading force and is permanently pressed against the toothing of the worm gear (or the helical gear). It has been shown that, in such designs, a change in rotational direction or a change in load in the gearing combined with existing gear play can cause the tooth faces of the helical pinion or the worm, and the tooth faces of the helical gear or the worm gear to impact one another, thereby generating impact noises. Nevertheless, the gear play should not be set too low during assembly since this can negatively influence the effectiveness and the wear of the gearing. In addition, the feedback properties of a power-assisted steering system can be impaired as a result, which, in turn, impairs the driver's steering sensation.
The maximum preload with which the worm can be pressed into the worm gear is limited since the worm gearing would jam otherwise.
The helical gears or worm gears used in electric steering systems are often made of plastic. Said plastic parts are subjected to creep deformation or wear throughout the service life. To ensure the elastically urging of the worm against the worm gear throughout the entire service life, a very large spring force, which diminishes over the course of the service life due to an increase in gear play, is applied in the new state in the case of the helical gearing or worm gearing known from the general prior art. In the new state, this results in increased friction and, therefore, efficiency disadvantages in the gearing.
Reference is made to DE 10 2008 000 506 A1 for the further prior art.